JP2004-342520A, published by the Japan Patent Office in 2004, proposes a secondary battery manufacturing method for manufacturing a secondary battery having a casing constituted by a thin, lightweight outer covering film such as a laminate film, an object thereof being to remove gas generated during conditioning and the like while ensuring that the casing is airtight following removal of the gas.
This conventional technique includes a sealing process in which a storage element is sealed in the interior of the casing constituted by the outer covering film, a degassing process in which gas is removed from the interior of the casing, and a second sealing process. In the sealing process, the casing is clamped by a jig, whereupon the outer covering film is joined apart from a non-joined portion set in a predetermined position of the casing. In the degassing process, a degassing hole is formed in the non-joined portion using a needle or the like while constraining a thickness of the non-joined portion to or below a predetermined thickness using the jig. In the second sealing portion, the degassing hole is closed by joining the non-joined portion.
In general, when the non-joined portion expands or deforms by a large amount due to an increase in internal pressure caused by the generated gas, the non-joined portion may not return to its original shape when joined. Moreover, when the non-joined portion expands, stress may be concentrated on the periphery of the non-joined portion, causing a part of the joined portion to peel back. The processes of the conventional technique described above have the effect of preventing these problems from occurring.
In the degassing process of the conventional technique, in which the gas generated in the casing is discharged to the outside, the degassing hole is formed in the non-joined portion. However, when electrolyte remains in the non-joined portion, the residual electrolyte in the non-joined portion flows to the outside of the casing together with the gas through the degassing hole, leading to a reduction in the amount of electrolyte in the casing. When the electrolyte flowing out of the casing adheres to the surface of the casing, a process for wiping the electrolyte away is required, and as a result, the production cost of the secondary battery may increase.